1. Field of the Invention
The present invention relates to the field of plant biotechnology. The present invention also relates to control of plant morphology and fertility with plant symbiotic bacteria that produce varying amounts of cytokinin.
2. Brief Description of the Related Art
Previously, it has been demonstrated that Pink-Pigmented Facultative Methylotroph (PPFM) bacteria, especially Methylobacterium spp.:                are distributed ubiquitously on plants (Corpe, W. A. & Basile, D. V. (1982) Dev. Indust. Microbiol. 23, 483-493; and Corpe, W. A. & Rheem, S. (1989) FEMS Microbiol. Ecol. 62, 243-250),        are present in large numbers (Hirano, S. S. & Upper, C. D. (1992) in Microbial Ecology of Leaves, eds. Andrews, J. H. & Upper, S. S. (Springer, New York), pp. 271-279),        stimulate plant growth in vitro (Basile, D. V., Slade, L. L., & Corpe, W. A. (1969) Bull. Torrey Bot. Club 96(6), 711-714; and Basile, D. V., Basile, M. R., Li, Q. Y., & Corpe, W. A. (1985) Bryologist 88(2), 77-81),        participate in plant nitrogen metabolism (Holland, M. A & Polacco, J. C. (1992) Plant Physiol. 98, 942-948; Stebbins, N. E., Holland, M. A., Cianzio, S. R., & Polacco, J. C. (1991) Plant Physiol. 97, 1004-1010)        enhance seed germination (Holland, M. A. & Polacco, J. C. (1994) Annu. Rev. Plant Physiol. Plant Mol. Biol. 45, 197-209)        stimulate root growth (Holland, M. A. (1997) Recent Res. Devel. in Plant Physiol. 1, 207-213)        
Most recently, we have demonstrated in soybean that foliar applications of the bacteria to plants under adverse field conditions can produce a 70% increase in seed yield over untreated plants, as described in U.S. Pat. No. 5,961,687, which is incorporated by reference herein in its entirety. The seemingly varied effects of this bacterium on plant performance are explained by its production of the plant growth regulator, cytokinin (Freyermuth, S. K., Long, R. L. G., Mathur, S., Holland, M. A., Holtsford, T. P., Stebbins, N. E., Morris, R. O., & Polacco, J. C. (1996) in Microbial Growth on C1 Compounds, eds. Lidstrom, M. E. & Tabita, F. R. (Kluwer, Dordrecht), pp.277-284). So, it is reasonable to consider other physiological effects of cytokinins on plants and to investigate whether PPFMs can also be used to alter them. One such physiological effect is on normal flower development and fertility.
Methylobacterium is a seed-transmitted bacterium. That is, the bacteria are incorporated into developing seeds and are reliably passed from generation to generation. Because of the way the bacteria are inherited, any trait they confer on their host plant will look like an example of “maternal” or “cytoplasmic” inheritance. A classic example of this pattern of inheritance is given by cytoplasmic male sterility, a trait which is exploited for the production of F1 hybrid seed. Several different and independently-operating mechanisms are known to account for cytoplasmic male sterility. Plant growth regulators, including cytokinin, are known to influence sex expression in many plant species (Frankel, R. & Galun, E. (1977) Pollination mechanisms, reproduction and plant breeding, Springer Verlag, Berlin. 218 pp).
Methylobacterium produces cytokinin and it passes from generation to generation in the same pattern as cytoplasmic genes. Interestingly, it has been shown in barley that cytoplasmic male sterile plants (CMS) are characterized by abnormally low levels of cytokinins (Ahokas (1982) Proc. Nat. Acad. Sci. USA. 79:7605-7608). If bacteria are an important natural source of cytokinins in plants (Holland, M. A. (1997) Plant Physiol 115, 865-968), then differences between bacterial strains with respect to cytokinin production could be responsible for a host of differences in growth and development between plant varieties—including fertility. Thus, different strains of Methylobacterium can be used to produce and correct male sterility in a variety of plants.